Vessels for holding liquids and pressure vessels are usually tested to determine their integrity before they are put into use. Leakage of fluid from a vessel can be due to many causes--for example, welds, fittings or valves may leak and, when higher pressures are used to store gases such as hydrogen or helium, even a porous weld or a porous metal plate can be a source of leakage. Integrity testing is normally accomplished by putting gas at elevated pressure into a vessel fitted with a pressure gauge. The pressure of the gas used to test the vessel is at or above the pressure expected to be the operating pressure of the vessel. The vessel being tested, hereinafter the test vessel, is pressurized to the test pressure after which all valves are closed and the pressurized vessel is permitted to stand for an appropriate time period. The pressure indicated by the pressure gauge is noted and recorded, and loss of pressure over time is an indication that the vessel leaks. This method of testing a vessel has many drawbacks because, particularly at high test pressures, small losses of pressure are hard to discern on a gauge that is capable of indicating the test pressure.
It is known to test the integrity of a vessel by measuring the difference in pressure between the test vessel and a reference vessel which usually has a much smaller volume and is constructed to have virtually no leakage. The integrity of the test vessel is tested by pressurizing both the test vessel and the reference vessel with exactly the same gas pressure, usually from a common source of gas, and then isolating the test vessel from the reference vessel and isolating both vessels from the common source of gas. In such a system leakage is found by the pressure difference between the two vessels after they have stood for a sufficient time period. With this method of testing, leakage is indicated by small pressure differences even though the absolute pressure in both vessels may be very large; therefore it is a much more sensitive test than just measuring gross pressure-loss. French patent No. 978,221 issued to Sterrett discloses such a process and apparatus.
However, pressure differences occur for reasons other than leakage of gas from a test vessel. In almost all pressure tests the pressurized gas in the test vessel is initially at a different temperature than the ambient temperature. When test gas is introduced into the test vessel from a high pressure source such as a gas bottle, the expansion of gas causes it to cool to a temperature lower than ambient temperature. When gas is introduced into the test vessel directly from a compressor, it is hotter than ambient because of the heat caused by compressing it. Usually the test vessel is much larger in volume than the reference vessel so that initial gas temperatures in the two vessels will be different. In addition, factors such as oxidation of the vessel walls, expansion of vessel walls or adsorption of gas (generally called seasoning of the vessel) can cause pressure changes within a test vessel due to factors other than leakage. As a result, in almost every pressure test using a test vessel and a reference vessel, either the test must be carried on for a long enough time period for pressure-influencing factors other than leakage to stabilize, or a subjective judgment must be made that observed pressure-differences between the vessels are caused by factors other than leakage.